Situ method and apparatus for biodegradation of alkyl ethers and tertiary butyl alcohol

ABSTRACT

A method and apparatus for the in situ purification of aquifers contaminated with ethers and/or alcohols using a bacterial culture, a method of delivering the culture to the subsurface, and an oxygen delivery system.

This Application claims the benefit of U.S. Provisional ApplicationSerial No. 60/29328, filed Apr. 14, 1999, the entire disclosure of whichis incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the in situ purification of aquiferscontaminated with oxygenate chemicals such as alkyl ethers and tertiarybutyl alcohol. This invention further relates to a method and apparatusthat cause the alkyl ethers and tertiary butyl alcohol to be biodegradedin Situ to carbon dioxide and water. In particular, the inventionrelates to the use of a bacterial culture, a method of delivery and/orbacterial stimulation in the subsurface, and an oxygen delivery systemfor the remediation of aquifers contaminated with methyl-t-butyl ether(MTBE).

BACKGROUND OF THE INVENTION

The 1990 Clean Air Act Amendments mandated that gasoline suppliersreformulate their gasoline blends to be cleaner burning, and to haveless impact to the atmosphere. In response to this mandate, gasolinesuppliers began to blend their fuels with oxygenate chemicals, such asalkyl ethers. In particular, methyl-tert-butyl ether (hereinafter“MTBE”), was used quite extensively, and often comprised up to 10 to 15%by volume of unleaded gasoline.

Now, having used oxygenated fuels for several years, it has become clearthat these cleaner-burning fuels pose great threats to groundwaterresources. In particular, many oxygenate chemicals are very soluble inwater and are slow to degrade in the environment; hence they tend toaccumulate in water resources once released to the environment. Forexample, MTBE has been detected in groundwater with high frequency inmany states and there are well-documented cases of impacts to municipalwater supply wells. In some cases these impacts result from accidentalgasoline spills; in other cases they are attributed to the re-depositionof chemicals emitted to the atmosphere from partially combustedautomobile exhaust.

It is also now known that when oxygenate chemicals including alkylethers, such as MTBE and tertiary butyl alcohol (hereinafter “TBA”), arefound in the subsurface, then they are resistant to biodegradation undernatural conditions. This is the main reason for their persistence andaccumulation in soil and groundwater.

With the increase in our knowledge of the behavior of these chemicals,and the increase in documented impacts, regulatory agencies are nowvigorously enforcing cleanup standards for MTBE and TBA in groundwater.As a result, practitioners are searching for technologies that canclean-up soil and aquifers contaminated with MTBE and TBA.

Shallow contaminated soil can be treated by excavating the contaminatedsoil and then treating it above-ground. However, in most cases, it ispreferable to treat contaminated soils in situ so as to minimizedisturbance of the site and prevent further release of the contaminantsto the atmosphere. Along these lines, the invention described by Visseret al. (U.S. Pat. Nos. 4,593,760 and 4,660,639) has been used at someMTBE-impacted sites with success. Visser's process relates to theextraction of soil gas vapors from the subsurface. However, this processis limited in applicability to permeable soils located above the watertable, and is limited to volatile alkyl ethers. Furthermore, by itself,it is a non-destructive process that must be coupled with above-groundtreatment (such as thermal oxidation) if the alkyl ethers or TBA are tobe destroyed.

While there is little performance data available, it is thought bypractitioners that contaminated groundwater might be remediated byapplication of either pump-and-treat technology or in situ air spargingtechnology. In the former, contaminated groundwater is withdrawn bypumping from groundwater wells and is purified above-ground. Followingpurification, the groundwater is reinjected to the aquifer or dischargedabove-ground. A groundwater pump-and-treat system is viewed as bothineffective and expensive because it is maintenance-intensive,operations often are on the scale of decades, and it merely transferscontamination from the aqueous phase to the atmosphere, to a solidmedium for later disposal, or to a surface treatment facility. In situair sparging technology is described by Billings et al. (U.S. Pat. No.5,221,159). In that invention, air is injected in situ into thecontaminated groundwater with the hope that groundwater contaminantswill be volatilized or that the addition of oxygen will help the aerobicbiodegradation of readily biodegradable contaminants. With respect toapplication to oxygenate chemicals such as alkylethers and TBA,indigenous organisms capable of biodegrading these chemicals are notalways present; if they are present, it is usually at such low numbersthat the in situ air sparging process can practically only cause thevolatilization of the alkylethers and TBA. Thus, application of thisprocess would only cause the transfer of alkylethers and TBA from thegroundwater to soil gas and the atmosphere. In some cases the vapors arecollected and treated above-ground, but again, above-ground treatment ofvapors is typically very expensive and problematic.

U.S. Pat. No. 5,874,001, issued Feb. 23, 1999, proposes a method forremoving contaminants from ground water or soil by injecting oxygen gasinto ground water.

To date, there is no in situ treatment process known to the practicethat result in a satisfactory in situ destruction of the alkyethers andTBA. There is need, therefore, for a practicable in situ technology thatresults in the satisfactory destruction of the target alkyl ether(s) insitu, and that does not require the withdrawal and above-groundtreatment and discharge of fluids.

U.S. Pat. Nos. 5,750,364 and 5,811,010, assigned to Shell Oil Company,relate to a bacterial culture that aerobically degrades alkylethers andTBA to non-toxic carbon dioxide and water. However, in-situ remediationof sub-surface chemical contaminants by delivering contaminant-degradingbacteria to the sub-surface (this overall process will hereinafter bereferred to as “bio-augmentation”) has not historically been embraced bythe practice. This is mainly because most soils and aquifers alreadyhave sufficient numbers of microorganisms capable of causing substantialbiodegradation of readily biodegradable compounds, and thereforeaddition of other organisms is not warranted. It is also accepted thatdelivering and maintaining non-indigenous microorganism cultures in thesubsurface is a very difficult task.

Now, with the need to treat aquifers and soils contaminated with morerecalcitrant chemicals, there is a need for a bio-augmentation processwhich can successfully deliver and maintain non-indigenous microorganismcultures in the subsurface. More specifically, there is a need for aneffective bio-augmentation process for remediating oxygenate chemicalssuch as alkyl ethers, particularly MTBE, and TBA contamination in soilsand groundwater.

SUMMARY OF THE INVENTION

This invention relates to a method and apparatus for the in situbioremediation of aquifers contaminated with chemical contaminant(s) byinjecting into the aquifers a microbial culture that degrades thecontaminants(s). This invention further relates to a method andapparatus for the in situ bioremediation of aquifers contaminated withoxygenate chemicals such as alkyl ethers, such as MTBE, and/or t-butylalcohol (TBA) by injecting into the aquifers a microbial culture thatdegrades MTBE and/or t-butyl alcohol. In particular, the invention uses(a) a bacterial culture capable of aerobically degrading the targetchemicals, (b) a method for delivering the mixed culture to thesubsurface with sufficient degrading activity, and (c) an oxygendelivery system injecting, by means of a network of at least twoconduits which extend below the treatment zone, an oxygen-containing gasat a pressure of at least 5 psig (pounds per square inch gauge) abovethe hydrostatic pressure at each point of delivery, by pulsed injection,at a frequency in the range of from about once per week to about 10times per day optimized so as to maximize aerobic biodegradation whilemaintaining less than 50%, preferably less than 10% volatilization ofcontaminants.

To reach the optimal delivery of the oxygen-containing gas, theinjection frequency and volume at each injection point are adjusted tohave the relationship according to the following equation:

e ^([(−V×F×N×H)/(W×B×Q)])>0.50 (preferably >0.80, more preferably >0.90,still more preferably >0.93)

Wherein:

e=natural exponential

V=volume of gas injected at each injection point (ft3)

F=frequency of injections (number of injections per day)

N=number of gas injection points

W=width of treatment zone perpendicular to groundwater flow path (ft)

B=vertical thickness of treatment zone (ft)

Q=specific discharge of groundwater to treatment zone (ft/day)

H=Henry's Constant for contaminant of interest ((mg/L-water)/(mg/L-air))

BRIEF DSCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of this invention.

FIG. 2 is a plan view drawing of an embodiment of this invention.

FIG. 3 shows a general lay-out of the remediation scheme/plot of oneillustrative embodiment of the present invention.

FIG. 4 illustrates the reduction of MTBE concentration in shallowmonitoring points using the process of the present invention.

FIG. 5 illustrates the reduction of MTBE concentration in deepmonitoring points using the process of the present invention.

FIG. 6 illustrates the increases in dissolved oxygen throughout thetreatment zone.

FIG. 7 illustrates the effect of oxygen-containing gas injections onMTBE concentrations without the inoculation of MTBE degradable culture.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for the in-situ remediation ofsub-surface chemical contaminants by delivering a bacterial culturecapable of degrading the contaminants at a sub-surface contaminatedsite. This process successfully delivers and maintains microorganismcultures in subsurface utilizing a network/galleries of culture and/oroxygen-containing stream delivery apparatus.

The present invention also provides a process for remediating in-situoxygenate chemicals including alkyl ethers and alkyl alcohols,particularly branched alkyl ethers/alcohol's, more particularly tertiaryalkyl ethers/alcohols, still more particularly MTBE and TBAcontamination in soils and groundwater by delivering to the aquifersand/or soils a bacterial culture capable of degrading such oxygenatechemicals, particularly branched alkyl ethers/alcohols such as MTBEand/or TBA, preferably to carbon dioxide and water, more preferably tocarbon dioxide and water within 70 hours. The bacterial culture can be,but not limited to, an indigenous culture, collected and optionallyenriched and/or grown to a larger quantity, from the contaminated site.Non-limiting and illustrative examples of the oxygenates include diethylether (DEE), dimethyl ether (DME), methyl ethyl ether (MEE), methyln-propyl ether (MPE), ethyl n-propyl ether, methyl isopropyl ether,ethyl isopropyl ether, di-t-butyl ether, di-isopropyl ether (DIPE),di-isobutyl ether, isopropyl isobutyl ether, ethyl t-butyl ether (ETBE),t-amyl ethyl ether, t-amyl propyl ether, t-amyl isobutyl ether ormethyl-t-amyl ether. As one aspect of the present invention, thebacterial culture delivered is a non-indigenous culture.

Applicants have discovered by surprise that the remediation can beachieved with great success by using a network of oxygen delivery wellsoperated intermittently so as to maximize aerobic biodegradation withoutcausing significant losses from other non-degrading mechanisms (e.g.volatilization, dispersion, etc.) Referring to FIGS. 1 and 2, theinvention comprises one or more sources of oxygen-containing gas 1connected to one or more subsurface gas injection points 20 viainjectors 9. Oxygen-containing gas delivery is used to create one ormore well-oxygenated zones 12 in the aquifer 14 conducive to aerobicbiodegradation. Within these aerobic zones, a bacterial culture 11capable of degrading the oxygenate chemical contaminant(s), specificallyalkyl ethers/alcohols, more specifically MTBA and/or TBA is delivered.Contaminated groundwater 13, specifically contaminated with alkyl ethers(such as MTBE) and/or TBA flows from contaminated aquifer soils 15 tothe treatment zones 23. As contaminated groundwater passes through thistreatment zone, it is purified by biodegradation, and the toxic alkylethers/alcohols such as MTBE and TBA are converted to non-toxicsubstances such as carbon dioxide and water.

Also referring to FIGS. 1 and 2, the oxygen-containing gas sources 1 maybe housed in enclosures 18. The oxygen-containing gas sources 1 may becomprised of air compressors, blowers, or combinations thereof withpressurized gas storage tanks. The oxygen gas sources 1 may also becomprised of commercially-available oxygen gas generators.

The oxygen-containing gas delivery systems must be designed so as tomaintain relatively uniform aerobic conditions in the treatment zones23, without causing significant contaminant loss throughnon-biodegradation loss mechanisms, such as volatilization anddispersion. The loss of contaminant(s) from volatilization (such asmigration to the unsaturated zone) is typically less than 50%,preferably less than 20%, more preferably less than 10%, and still morepreferably less than 7% by weight.

One non-limiting way for achieving this involves the intermittent pulsedinjection of an oxygen-containing gas, such as air or pure oxygen. Thus,pulses of oxygen-containing gas are delivered through the injectors 9.When operated in this fashion, a portion of the oxygen-containing gasbecomes trapped in the aquifer pore spaces and oxygen continuouslydissolves into groundwater during the time period between injectionpulses. Thus, oxygen is continuously delivered between pulses. Inaddition, pulsed gas delivery is critical as continuous delivery cancause reductions in water permeability to the extent that contaminantsflow around, rather than through, the treatment zone. FIG. 1 depicts oneembodiment of an apparatus for delivering oxygen-containing gas in anoptimized intermittent mode. First, the desired injection pressure ofthe oxygen-containing gas is controlled with gas pressure regulators 2.The injection pressure must be greater than the hydrostatic head ofwater in the injectors 9; which is roughly 0.5 psig per foot of depthbetween the water table elevation and the top of the perforations in theoxygen delivery well. To eliminate the poor gas distribution problemsthat plague manifolded gas delivery wells, it is important that thedelivery pressure be at least 5, and preferably at least 10 psig greaterthan the hydrostatic head of water. Illustrative examples of delivery

pressures are in the 20-40 psig range. One or more gas storage tanks 5,are used to store the oxygen-containing gas between injection cycles.The gas storage tanks 5 are sized to contain the desired gas injectionvolume at the desired gas injection pressure. The gas storage tanksallow the short and rapid pulsed delivery of gas not achievable by wellsdirectly connected to blowers, compressors, or other oxygen-generatingequipment. Solenoid valves 3 and back-flow check valves 4 control theflow of gas from the oxygen-containing gas source 1 and to the injectors9.

System monitoring and control is integral to optimal operation of thisinvention. System monitoring is typically comprised of a collection ofpressure gauges 7 and gas flow meters 6 that are used to ensure gasdelivery to each injector is occurring. At a minimum, pressure gaugesare needed for each gas storage tank. In addition, groundwatermonitoring points 10 and dissolved oxygen sensors 8 are used to monitordissolved oxygen concentrations in the target treatment zone. Based onthis data, the frequency and volume of oxygen-containing gas injectionsis adjusted through use of the controller 19. This controller may be asimple system of manual valves, but more often will involve electronictimers, and may even involve more complicated automatic control systemsintegrated with in situ oxygen sensors.

Gas injectors 9 are typically, but not limited to, constructed from acombination of perforated 22 and non-perforated conduit 21. Thenon-perforated conduit 21 must extend to below the target treatment zone23, and must be installed so as to prevent the short-circuiting ofoxygen-containing gas to the vadose zone 17. The perforated section 22of the injector 9 must be placed below the target treatment zone, butmay be aligned either vertically or horizontally. If aligned verticallyas shown in FIG. 1, the perforated section is usually not much longerthan a few feet.

As one aspect of the present invention, the dissolved oxygenconcentration (DOC) in the aquifer treated (treatment zone) is increasedto greater than 2 mg/L in the aquifer (groundwater) treated zone,preferably greater than 4 mg/L and more preferably more than 8 mg/l, andstill more preferably greater than 18 mg/L within less than 40 Days,preferably within less than 30 days, more preferably within less than 20days, still more preferably within less than 14 days of gas injection.Dissolved oxygen concentrations up to about 8 g/L can be achievedthrough air or oxygen gas delivery; igher dissolved oxygenconcentrations require pulsed xygen gas delivery.

The pressure of the oxygen-containing torage tanks is at least 5,preferably 10, more preferably more than 20 psig, and still morepreferably more than 30 psig above the hydrostatic pressure at theinjection point. As a non-limiting illustrative example, the hydrostaticpressure at the injection point is calculated to be about 0.5 psig perft. of depth below the water table. At a typical treatment zone, theinjection points are at the depth of about 5-20 ft, preferably about5-10 ft, below the water table. The hydrostatic pressure at theinjection points is 2.5-10 psig. The injection (storage tank) pressurecould then be at least 7.5-15 psig, and preferably greater than 22.5-30psig. The injection pressure can be 6-60 psig.

To ensure adequate oxygen delivery, the space between theoxygen-containing gas injection points is typically less than 20 ft, andpreferably less than 10 ft, and more preferably less than 7 ft, andstill more preferably less than 6 ft. As a non-limiting example, thespacing between adjacent injection points can be about 3-5 ft apart,particularly in a shallow (<30 ft depth), relatively homogenous aquifer.

As one aspect of the present process, the minimum average volume(Vmin—given in cubic feet of gas measured at ambient temperature andambient pressure) of total oxygen injected each time at each injectingpoint can be calculated by the following formula:

Vmin=0.1×A×B×P÷N

Wherein, A=treated area (square ft)

B=treatment thickness (ft)

P=porosity

N=number of injection points

The volume of oxygen injected is about 1-100, preferably 1.25-20, andmore preferably 1.25 to 3 times of “Vmin”. “P” is the porosity and istypically a value between about 0.25 to about 0.45 and the standardvalues of P vary with the aquifer material, which can be looked up inreference books generally known to one skilled in the art.

It is understood that when the oxygen-containing gas is not a relativelypure oxygen, such as air, the volume of gas injected shall be adjustedaccording to the oxygen content of the oxygen-containing gas.

As an illustrative non-limiting example, a 100 square ft by 12 fttreatment zone with 30 oxygen injection point will require at leastabout 4, preferably 5-8, more preferably 5-7 cubic feet(based onmeasurement at ambient pressure and temperature) of oxygen per injectionpoint.

The frequency for oxygen-containing gas injection will be variable andadjusted on a site specific basis. Typically it is from about once aweek to about 10 times per day, specifically from about once a day toabout 8 times a day, more specifically from about 2 times day to about 4times a day. The duration of each injection of oxygen-containing gas ateach injection points lasts from about 0.05 to about 4 minutes,preferably from about 0.1 to about 3 minutes, still more preferably fromabout 0.3 to about 2.0 minutes. As an illustrative non-limiting example,oxygen-containing gas is injected at each injection site at about 1 toabout 20, specifically from about 2 to about 10, more specifically fromabout 3 to about 8 cubic feet per minute (based on the volume at ambientpressure) for from about 0.1 to about 3 minutes. As a specificillustrative example, the oxygen-containing gas is injected at theinjection point (well) at about 4 cubic feet per minute i.e. 260 cubicfeet per hour) for 0.5 to 1 minute for each injection.

As an embodiment of the present invention, to maximize aerobicdegradation and not cause significant loss of contaminant(s) byevaporation and dispersion, the injection frequency and volume at eachinjection point have the relationship according to the followingequation:

e ^([(−V×P×N×H)/(W×B×Q)])>0.5

Wherein:

e=natural exponential

V=volume of gas injected at each injection point (ft3)

F=frequency of injections (number of injections per day)

N=number of gas injection points

W=width of treatment zone perpendicular to groundwater flow path (ft)

B=vertical thickness of treatment zone (ft)

Q=specific discharge of groundwater to treatment zone (ft/day)

H=Henry's Constant for contaminant of interest ((mg/L-water)/(mg/L-air))

Where conditions satisfy the equation above, less than 50% of thecontaminant is volatilized and/or dispersed by the gas delivery process.Preferably, e^([(−V×F×N×B)/(W×B×Q)]) is greater than 0.80, morepreferably greater than 0.90, and still more preferably greater than0.93.

As an illustrative example, the design and operating conditions in atest plot are as follows:

V=2.5 ft3/well

F=4 injections per day

N=21 wells

W=20 ft

B=10 ft

Q=0.3 ft/day)

H=0.02 (mg/L-water)/(mg/L-air)

Under these conditions:

e ^([−V×F×N×H)/(W×B×Q)])=0.93

Under the above conditions, the test plot can achieve adequateoxygenation for biodegradation while maintaining only less than 10% loss(or even less than 7% loss) of contaminants by evaporation and/orvolatilization.

The concentration of the culture delivered and contained in the treatedzone will depend on the strength and activity of the culture.

Where MTBE remediation using the mixed bacterial culture disclosed inU.S. Pat. No. 5,750,364, ATCC Number 202057, is conducted, theconcentration of the bacterial culture used is typically more than 100mg, more preferably greater than 200 mg, more preferably greater than250 mg, dry wt of cells/kg of soil. The culture is capable of degradingMTBE and TBA to carbon dioxide and oxygen.

The concentration of the culture contained in the injection slurry isgenerally within the range 100-10,000 mg/L, preferably about 200-5000mg/L, and still more preferably about 500-4000 mg/L.

The apparatus for delivering bacterial slurry can be, but not limitedto, injection wells, infiltration galleries e.g. via trenches or drains,direct injection under pressure through open-ended pipes. As a specificembodiment of the present invention, nutrients are also injected to thetreatment site to enhance biodegradation. The apparatus is designed todeliver bacterial slurry effectively to distribute well through thetreated zone in a fast enough fashion to prevent settling of the culturein the delivery system to prevent plugging of the apparatus, preferablysufficiently fast with sufficient pressure to reach the treatment areafar enough from the injection site, more preferably with sufficientpressure and/or speed to alter/fracture/crush the soil structure tocreate channels for the bacterial to reach out farther in the treatmentzone. As a non-limiting examples, where injection concentrations aremore than 1000 mg dry wt of cells per liter of solution, an open-endedpipe under pressure of at least 10 psig, more preferably at least 40psig, still more preferably more than 75 psig greater than thehydrostatic head pressure is used. For injection concentration of lessthan 1000 mg dry wt of cells per liter of solution, injection wells,infiltration galleries may suffice; if so, it is set-up typically in anet-work or galleries of delivery conduits. Injection of nutrientsand/or bacteria can be made through the oxygen-containing gas injectionwells, monitoring wells, or separate wells designed for bacterial slurryinjections only.

Groundwater monitoring points 10 are also constructed from perforatedand non-perforated conduit. In this case, it is important that theperforated conduit section be limited to the depth interval of interestfor monitoring. Dissolved oxygen sensors 8 may be placed in situ, in thegroundwater monitoring points, or above-ground. If above-ground, theywill be coupled with a groundwater pumping system for bringing smallquantities of groundwater above-ground from the groundwater monitoringwells 10. Groundwater monitoring points placed down-gradient of thetarget treatment zone will also be used to monitor the disappearance ofthe alkyl ethers and TBA.

Delivery of the bacterial culture 11 to the treatment zone may beachieved through: a) use of injection wells, b) infiltration viatrenches or drains, c) direct injection under pressure throughopen-ended pipes. It is critical that sufficient culture be deliveredinitially as it is not typically expected that the degrading activity inthe treatment zone will increase with time.

As one embodiment of the present process for remediating MTBE and/or TBAin the aquifer (groundwater, saturated zone, water-bearing zone,sub-soil), a mixed bacterial culture or pure bacterial culture describedin U.S. Pat. No. 5,750,364 and U.S. Pat. No. 5,811,010, assigned toShell Oil Company, as well as all subsequent continuation(-in-part)applications of Ser. No. 465,996 filed Jun. 6, 1996, is used; thedescriptions of these patents/applications are herein incorporated byreference.

The invention can be further demonstrated by the following non-limitingillustrative embodiments.

ILLUSTRATIVE EMBODIMENTS

The following illustrative embodiments describe one application of thisinvention.

I. Bioremediation of MTBE

This invention was tested at the pilot-scale at the U.S. Navy CBC Basein Port Hueneme, Calif. The general lay-out of the demonstration plot isshown in FIG. 3. Three test cells were constructed to evaluate theperformance of this invention relative to two controls. The center plotcontains the illustrative embodiment of this invention. The plots oneither side contain: a) a control plot (only monitoring), and b) a plotinto which only oxygen gas is injected.

At this site the target treatment zone is roughly 5-ft long×20-ftwide×12-ft deep. 1400 gallons of mixed culture BC-4 containing roughly2000 mg-TSS/L (total suspended solids per liter) were injected into thiszone through the open end of rigid pipe. The pipe was driven down to thebottom of the target treatment zone and then slowly raised as the mixedculture was injected under pressure. Injections were made every 1-ftalong the length of the treatment zone and approximately 5 gallons ofmixed culture were injected for every 1-ft rise of the injection pipe.

The oxygen delivery system consisted of roughly 40 oxygen injectorsdistributed as shown in FIG. 3. Each was constructed from non-perforated¾-inch diameter PVC pipe to which was attached either a 2-ft or 1-ftperforated section at the end. The injectors were installed usingcommercially-available direct-push well installation methods. Perforatedsections were either placed from 17-19 ft below ground surface(hereinafter referred to as “deep oxygen delivery wells”)., or from14-15 ft below ground surface (hereinafter referred to as “shallowoxygen delivery wells”). Groundwater monitoring wells were constructedand installed in the same fashion, except they had 5-ft perforatedsections placed at either 15-20 ft below ground surface (hereinafterreferred to as “deep wells”). or 10-15 ft below ground surface(hereinafter referred to as “shallow wells”).

The oxygen gas source was a commercially-available oxygen gas generatorcapable of generating up to 80 SCFM (standard cubic feet per minute). Itwas connected via a sequence of timers, pressure regulators, andsolenoid-valves to eight gas storage vessels. Each storage vessel had aninternal volume of roughly 2.5 cubic feet; when pressurized to 30 psig,each contained approximately 8 cubic feet of pure oxygen gas (based onvolume at ambient pressure and ambient temperature) Each storage vesselwas connected via electronic solenoid valves to three to five oxygendelivery wells. Timers were set so that oxygen gas could be deliveredanywhere from one to eight times per day. Each oxygen gas storage tankwas connected to 2 to 4 wells, and typically fully discharged in about0.5 to about 1 minute.

FIGS. 4a, 4 b, 4 c and 5 a, 5 b, 5 c display MTBE concentrations vs.distance from the top of each treatment plot for various times prior to,and after BC-4 injection. For reference, the times shown are measuredrelative to the date of BC-4 seeding; thus, t=−44 d corresponds to thestart of the O₂ delivery system and t=0 corresponds to BC-4 seeding.Each figure depicts a transect along the direction of groundwater flowfor one of the study plots (i.e., O₂ injection-only, BC-4+O₂ injection,and the control plot). For each distance along a given transect, thegeometric mean of the measured concentrations is displayed in FIGS. 4and 5.

For all the plots and all distances along the transects, MTBEconcentrations are relatively stable for all times prior to the BC-4seeding (t=0 in FIGS. 4 and 5). The dissolved oxygen concentrations inthe treatment zone reached over 20 mg/L (or 25 mg/L) after about 14 daysof oxygen injections.

Following BC-4 seeding and continued operation of the O₂ injectionsystem for the next 67 days, decreases in MTBE concentrations areobserved in the BC-4+O₂ injection plot. Immediately down-gradient of theBC-4 seeded zone MTBE concentrations decrease >90%, and other lessdramatic declines in MTBE were observed up-gradient of the seededregion. In comparison, no significant changes are observed in MTBEconcentrations in the O₂ injection-only or control plots.

In summary, the time series data clearly show the present invention,effectively provides a biobarrier to MTBE migration, and effectivelyreduces the MTBE contamination in the aquifer(s).

II. Oxyqen Delivery

A test plot was operated for at least 44 days with only oxygen gasdelivery (no inoculation of bacterial culture) under the followingconditions:

V=2.5 ft3/well

F=4 injections per day

N=21 wells

W=20 ft

B=10 ft

Q=0.3 ft/d(day?)

H=0.02 (mg/L-water)/(mg/L-air)

Under these conditions:

e ^([(−V×F×N×H)/(W×B×Q)])=0.93

Results from the treatment plot is shown in FIGS. 6 and 7. FIG. 6 showsthe increases in dissolved oxygen throughout the treatment zone in this44 day period; increases in excess of 10 mg/L were achieved throughoutmuch of the target zone. FIG. 7 shows the effect on MTBE concentrationsfor 76 days of gas injection. As can be seen, MTBE concentrations remainessentially unchanged, thereby providing evidence that volatilizationcould not be significant under these design and operating conditions.

What is claimed is:
 1. A method for in situ remediation of an aquiferhaving a treatment zone through which passes water contaminated with atleast one chemical contaminant which method comprises: a) delivering tothe treatment zone a microbial culture capable of degrading at least onechemical contaminant present in said aquifer; and b) injecting, by atleast two conduits, an oxygen-containing gas at a pressure of at least 5psig above the hydrostatic pressure at injection points with injectionfrequency and volume at each injection point having the relationshipaccording to the following equation: e ^([(−V×F×N×H)/(W×B×Q)])>0.50Wherein: e=natural exponential V=volume of gas injected at eachinjection point (ft3) F=frequency of injections (number of injectionsper day) N=number of gas injection points W=width of the treatment zoneperpendicular to groundwater flow path (ft) B=vertical thickness oftreatment zone (ft) Q=specific discharge of groundwater to the treatmentzone (ft/day) H=Henry's Constant for contaminant of interest((mg/L-water)/(mg/L-air)).
 2. The method of claim 1, wherein, whereine^([(−V×F×N×H)/(W×B×Q)]) is greater than 0.80.
 3. The method accordingto claim 1, wherein said contaminant is an oxygenate chemical; whereine^([(−V×F×N×H)/(W×B×Q)]) is greater than 0.90 and the contaminant lossfrom volatilization is less than 10% by weight.
 4. The method accordingto claim 1, wherein the contaminant is selected from the groupconsisting of (a) methyl-t-butyl ether (MTBE), (b) t-butyl alcohol(TBA), and (c) a mixture thereof; wherein at least a portion of thecontaminant is degraded to carbon dioxide by said microbial culture. 5.The method according to claim 1, wherein the dissolved oxygenconcentration in the treatment zone is greater than 2 mg/L.
 6. Themethod according to claim 1, wherein the dissolved oxygen concentrationin the treatment zone is greater than 8 mg/L.
 7. The method according toclaim 1, wherein the dissolved oxygen concentration in the treatmentzone is greater than 18 mg/L.
 8. The method according to claim 1,wherein said bacterial culture is delivered to the treatment zone by anetwork of a plurality of delivery conduits at a pressure of at least 10psig greater than the hydrostatic head pressure at point(s) ofinjection.
 9. The method according to claim 1, the oxygen-containing gasis injected to the aquifer by at least two gas injectors havingrespective perforated sections located at at least two different depthswithin the aquifer.
 10. The method according to claim 1, which methodfurther comprises storing the oxygen-containing gas in at least one gasstorage tank and subsequently delivering the oxygen-containing gas to atleast one sub-surface injection point by intermittent pulsed injection;and maintaining relatively uniform aerobic conditions in the treatmentzone by adjusting the volume per injection and the injection frequency,and by monitoring the oxygen flow from each gas storage tank to eachsub-surface gas injection point using at least one pressure gauge and/orgas flow meter, monitoring dissolved oxygen concentration in thetreatment zone using ground water monitoring points and dissolved oxygensensors, and adjusting the frequency and volume of each injection ofoxygen-containing gas using a controller associated with said monitoringequipment.
 11. The method as claimed in claim 10, which method furthercomprises storing the oxygen-containing gas in at least two gas storagetanks, each storage tank being connected to at least one sub-surfaceinjection point.
 12. The method as claimed in claim 1, wherein saidmicrobial culture and said oxygen-containing gas is delivered throughthe same conduit(s) to the treatment zone.
 13. A method for in situremediation of oxygenate chemical(s) in an aquifer having a treatmentzone through which passes water contaminated with oxygenate chemicals,which method comprises: a) delivering to the treatment zone a microbialculture capable of degrading at least one oxygenate chemicalcontaminant; and b) injecting, by at least two conduits, anoxygen-containing gas at a pressure of at least 5 psig above thehydrostatic pressure at injection points with injection frequency andvolume at each injection point having the relationship according to thefollowing equation: e ^([(−V F×N×H)/(W×B×Q)])>0.90 Wherein: e=naturalexponential V=volume of gas injected at each injection point (ft3)F=frequency of injections (number of injections per day) N=number of gasinjection points W=width of treatment zone perpendicular to groundwaterflow path (ft) B=vertical thickness of treatment zone (ft) Q=specificdischarge of groundwater to treatment zone (ft/day) H=Henry's Constantfor contaminant of interest ((mg/L-water)/(mg/L-air)) Wherein the lossof contaminant from volatilization and/or dispersion is less than 10% byweight; wherein the oxygen-containing gas is injected to the aquifer byat least two gas injectors having respective perforated sections locatedat least two different depths within the aquifer; wherein theoxygen-containing gas is injected to the aquifer by a plurality of gasinjectors spaced less than 10 ft apart; wherein each injection ofoxygen-containing gas at each injection point lasts from about 0.05 toabout 4 minutes.